I think if you need (or expect to need) more than 3GB of memory in the future, Vista (x64) is about the only viable option for a home user. That is, unless you can afford the licensing cost of the 64-bit Server 2003/2008 OS. So that is something Vista gives you that XP can't -- support for memory expansion. There is XP 64-bit but the less said about that the better. Given the work I do that requires at least 8GB, I had no other choice but to use Vista and it works perfect.
8GB makes me a minority, sure... for now, but it's not at all uncommon to see mid to high end off-the-shelf notebooks already kitted out with 3GB of memory. If they had any more, a downgrade to 32-bit XP would just be a waste of hardware since it can't adequately use more than 3.